Why do I keep imagining him cheating on me when I know he isn't?

Could be based on your past experience, that you have the trauma that he will cheat on you. But in all relationships u need trust. If there is no trust there is no foundation for you guys to have a relationship on.

It could be because your self-esteem is low, and your self-image is deteriorated by this. In this situation, I see just two cases that could have this effect: whether you feel that you are not sexually good enough in the relationship or as a person in whole. Maybe the communication is poor, and the feedback that you get from him not enough to clarify those two aspects, I mention above. And from this, you may think that he is cheating you because you are not good enough for him. And the fact that you are ignoring the real facts, it could be caused by the fact that we are driven by emotion not reason. This is the way I see this problem.

It seems to me like you have trust issues, and fear that other women/men are better than you. You should talk about your fear with your loved one, get couples therapy to help tighten the relationship and be true to one another.

when we love someone don't we fear that we lose them ? We feel attached to others we worry we might not see them often or we might lose their love so we think of the worst that can happen :)

I think deep down you know that i am not good enough for him or that your relation is a mismatch thats why you think that

It's probably your insecurities and fears just banging on the door. Also maybe the luck of trust in your partner.

It's Okay to have such fantasy because human beings are complicated.The feeling of uncertainty and anxiety will reflect on the expectation on others. For instance, I used to imagine my partner hurt me to gain a sense of security, so that I can experience the painful feeling in advance in my imagination.That was safe for me because it was only imagination. So I will be mentally prepared when I have to face the real hurtful situation. For me, it's a self-protection. I also talked to my friends a lot about similar topics. They also said they had similar imagination that their love ones hurt them badly. And the reason behind that, we all agree is that we are too afraid to lose them.

It sounds like you're worried and need some reassurance. Have you talked to him about this? That could be helpful.

You're afraid of losing him. That's normal and it means that he is important to you. You may talk with him about this.